(1) Field of the Invention
The present invention relates to a method of driving a DC motor in a printer operating a drive member by using the DC motor and, more particularly, to an improvement in the method of starting the DC motor.
(2) Description of the Related Art
FIG. 2 shows an example of a standard printer performing a printing operation using a type face belt. A worm 4 is fixed to an output shaft 3 of a DC motor 2 fixed to a frame 1. There are two idle gears 5 and 6 engaging with each other. One idle gear 5 is engaged with the worm 4, whereas the other idle gear 6 is engaged with a planetary gear mechanism (not shown) disposed at the left end of the frame 1. A main gear 7 and a drive pulley 8 rotatable together with a ratchet (not shown) are coaxially attached to the planetary gear mechanism. Cam recesses 9 are formed at the outer periphery of the main gear 7 so as to diametrically face each other.
A clutch gear 11 of an electromagnetic clutch 10 is engaged with the idle gear 6. A yoke 12 is coaxially attached to the electromagnetic clutch 10 and rotated by the rotation of the clutch gear 11. An electromagnetic plate 13 is disposed under the yoke 12 and drawn toward the yoke 12 when electricity is supplied to an electromagnetic coil (not shown) inside the yoke 12. A mating hole is formed in the electromagnetic plate 13.
A selection lever 15 branching out into three directions is rotatably attached to the shaft of the idle gear 6. A mating pawl 16a or 16b (only the pawl 16b being shown in FIG. 2) is formed at the end of either of the two portions of the selection lever 15 so that these pawls 16a and 16b are selectively engaged with the cam recess 9 in the main gear 7 and the tooth of the ratchet. A projection 17 is formed at the end of the one remaining portion of the selection lever 15 so that it is inserted into the mating hole 14 in the electromagnetic plate 13.
A lever spring 18 is attached to the selection lever 15, and used for urging the lever 15 counterclockwise as viewed in FIG. 2. One end of the lever spring 18 is fixed to the frame 1. The force exerted on the lever spring 18 maintains the selection lever 15 such that one engaging pawl 16b is engaged with the cam recess 9 in the main gear 7 and the other engaging pawl 16a is disengaged from the tooth of the ratchet when the lever 15 is at its home position.
The drive pulley 8 faces one side of the frame 1, whereas a driven pulley 19 faces the other side of the frame 1. A type face belt 21 is wound around the drive and driven pulleys 8 and 19, respectively, and serves as a type face supporting member in which a large number of type faces 20 are projected at certain intervals from the outer peripheries of the drive and driven pulleys 8 and 19, respectively.
A bevel gear-shaped digit-carrying gear 23 is engaged with the main gear 7 and attached to one end of a drive shaft 22 extending along the length of the frame 1. A hammer holder 24 for accommodating and retaining a hammer (not shown) is disposed inside the type face belt 21 so that it can be slid along the axis of the frame 1. The rotation of the drive shaft 22 operates the hammer.
A plate-like platen 25 supported by the frame 1 is disposed in front of the type face belt 21. A pair of mask members 26 are arranged between the type face belt 21 and the platen 25 so that there is a space substantially equal to the size of one type face 20 between the mask members 26. The mask members 26 are used for preventing the printing of the adjacent undesired type faces when the hammer presses the desired type face on the belt 21 against the platen 25.
A spring member 27 returns the hammer holder 24 to the home position when it is not operated.
A paper feed roller 28 is positioned under the type face belt 21 and the hammer holder 24 and rotatably supported by the frame 1. A pressure roller 29 is disposed over the paper feed roller 28 and used for retaining paper between the paper feed roller 28 and the pressure roller 29.
An ink roll 30 for applying ink to the type faces 20 on the type face belt 21 is disposed near the driven pulley 19.
To select a desired type face 20 in the thus-constructed printer, the DC motor 2 is rotatively driven by a DC signal, thereby rotating the worm 4 and the drive pulley 8 with the aid of the idle gears 5 and 6. This operation drives the type face belt 21 so that the desired type face 20 is moved to the corresponding digit.
When the desired type face 20 reaches the corresponding digit, electricity is supplied to the electromagnetic coil of the electromagnetic clutch 10, thereby suctioning the electromagnetic plate 13 toward the yoke 12. This operation rotates the yoke 12 and the selection lever 15 against the force acting on the lever spring 18. The engaging pawl 16a of the selection lever 15 engages with the ratchet, thus stopping the ratchet and then the type face belt 21. Simultaneously, the engaging pawl 16b of the selection lever 15 is disengaged from the cam recess 9 in the main gear, thus starting the rotation of the main gear 7. Under this condition, the supply of electricity to the electromagnetic coil is stopped.
The digit-carrying gear 23 is rotated as the main gear is rotated, thereby operating the hammer so that the desired type face 20 is pressed against the paper. The ink is transferred from the ink roll 30 to the paper, thus printing the desired type face 20.
When the main gear is further rotated after the desired type face has been printed, the engaging pawl 16b is moved to the cam recess 9 in the main gear 7. The selection lever 15 is rotated by the force exerted on the lever spring 18. This causes not only the engaging pawl 16b of the selection lever 15 to engage with the cam recess 9 in the main gear 7, but also the engaging pawl 16a to be disengaged from the tooth of the ratchet.
The above-described operation is repeated to select the next desired type face, to print it and to carry the digit. This operation is repeatedly performed as many times as the number of digits so that the printing of one line is completed. Upon the completion of the printing of one line, the spring member 27 returns the hammer holder 24 to its home position. The paper feed roll 28 and the pressure roll 29 move the paper in an amount equal to one line so that the next line can be printed.
During the printing operation, when the printer is stopped under a maximum load due to a power failure or a voltage drop in the battery, it requires a large amount of torque to restart the DC motor 2. The reason for this is that molded components stick together when they are compressed.
In order for the DC motor 2 to be restarted with such a maximum load, the following countermeasures are conventionally taken. First, the reduction ratio is increased and the rotation speed of the DC motor 2 is accordingly increased; second, the DC motor 2 is made larger; third, a DC motor having a high torque is employed; and fourth, an entrance period for the DC motor 2 is provided which period serves as a preliminary step for the printing operation.
When the above-mentioned countermeasures are taken, the various problems may occur as follows:
When the first countermeasure is taken, the amount of electric current as well as noise are increased.
When the second countermeasure is taken, it is difficult to further downsize a type face micro printer which is already miniaturized.
When the third countermeasure is taken, an expensive motor must inevitably be employed.
When the fourth countermeasure is taken, the number of components is increased and the reliability of the printer is decreased.